Optical glass, press-molding glass gob and optical element

ABSTRACT

An optical glass whose refractive index is high and whose coloring is decreased comprising, by weight %, 2 to 45% of B 2 O 3 , 0 to 30% of SiO 2  provided that the content of B 2 O 3 &gt;the content of SiO 2 , 10 to 50% of La 2 O 3 , 0 to 30% of TiO 2 , 0 to 15% of ZnO, 0 to 15% of ZrO 2 , 0 to 35% of Nb 2 O 5 , 0 to 35% of BaO, 0 to 5% of SrO, 0% or more but less than 8% of CaO, 0% or more but less than 13% of MgO, provided that the total content of BaO, SrO, CaO and MgO is 0 to 40%, 0 to 20% of Gd 2 O 3 , 0 to 15% of Y 2 O 3 , 0 to 18% of Ta 2 O 5 , 0% or more but less than 0.5% of WO 3 , 0% or more but less than 1.5% of a total of Na 2 O, K 2 O and Li 2 O, 0 to 10% of GeO 2 , 0 to 20% of Bi 2 O 3 , 0 to 10% of Yb 2 O 3 , 0 to 10% of Al 2 O 3 , 0% or more but less than 2% of Sb 2 O 3  and 0 to 1% of SnO 2 .

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of Ser. No. 11/140,911filed Jun. 1, 2005, now abandoned, which in turn is a divisional ofapplication Ser. No. 10/744,074 filed Dec. 24, 2003, now U.S. Pat. No.6,912,093, which in turn claims the priority of Japan Application No.2002-381147, filed 27 Dec. 2002. The entire contents of theseapplications are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to an optical glass, a press-molding glassgob (a glass gob for press-molding) and an optical element. Morespecifically, the present invention relates to an optical glass whoserefractive index is high and whose coloring is decreased, apress-molding glass gob formed of the optical glass and an opticalelement formed of the optical glass.

TECHNICAL BACKGROUND

In recent years, with the wide spread of digital cameras, small lensesare increasingly demanded. A high-refractivity glass is suitable as anoptical glass material for producing such small lenses. However, anyconventional glass has a disadvantage that the coloring tendency of theglass is increasingly intensified with an increase in refractive index.Particularly, a digital camera uses CCD as an image-sensing device, andit therefore has a problem that the sensitivity to blue on the shortwavelength side out of three primary colors is attenuated when an entireimage-sensing unit is taken into account. JP-A-53-4023 discloses ahigh-refractivity low-dispersion optical glass as a glass for use in theabove field. The problem with this glass is that it is required to useexpensive HfO₂.

SUMMARY OF THE INVENTION

Under the circumstances, it is an object of the present invention toprovide an optical glass whose refractive index is high and whosecoloring is decreased, a press-molding glass gob formed of the aboveoptical glass and an optical element formed of the above optical glass.

For achieving the above object, the present inventor has made diligentstudies and as a result has found that the above object can be achievedby an optical glass having a specific glass composition. The presentinvention has been completed on the basis of the above finding.

That is, the subject matters of the present invention are as follows.

(1) An optical glass comprising, by weight %, 2 to 45% of B₂O₃, 0 to 30%of SiO₂ provided that the content of B₂O₃>the content of SiO₂, 10 to 50%of La₂O₃, 0 to 30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂, 0 to 35%of Nb₂O₅, 0 to 35% of BaO, 0 to 5% of SrO, 0% or more but less than 8%of CaO, 0% or more but less than 13% of MgO, provided that the totalcontent of BaO, SrO, CaO and MgO is 0 to 40%, 0 to 20% of Gd₂O₃, 0 to15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more but less than 0.5% of WO₃, 0%or more but less than 1.5% of a total of Na₂O, K₂O and Li₂O, 0 to 10% ofGeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0 to 10% of Al₂O₃, 0% ormore but less than 2% of Sb₂O₃ and 0 to 1% of SnO₂.

(2) The optical glass of above (1), which has an refractive index (nd)of 1.8 to 2.1 and an Abbe's number (νd) of 20 to 40.

(3) The optical glass of above (1), which contains 2 to 45% of B₂O₃, 0to 30% of SiO₂, provided that the content of B₂O₃>the content of SiO₂,10 to 50% of La₂O₃, 0 to 30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂,0 to 35% of Nb₂O₅, 0 to 35% of BaO, 0 to 5% of SrO, 0% or more but lessthan 8% of CaO, 0% or more but less than 13% of MgO, provided that thetotal content of BaO, SrO, CaO and MgO is 0 to 40%, 0 to 20% of Gd₂O₃, 0to 15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more but less than 0.5% of WO₃,0% or more but less than 1.5% of a total of Na₂O, K₂O and Li₂O, 0 to 10%of GeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0 to 10% of Al₂O₃, 0% ormore but less than 2% of Sb₂O₃ and 0 to 1% of SnO₂, and has a refractiveindex of more than 1.86 but up to 2.1, wherein the optical glassexhibits λ₇₀ at 460 nm or less.

(4) The optical glass of above (1), which contains 2 to 45% of B₂O₃, 0to 30% of SiO₂, provided that the content of B₂O₃>the content of SiO₂,10 to 50% of La₂O₃, 0 to 30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂,0 to 35% of Nb₂O₅, 0 to 35% of BaO, 0% or more but less than 2% of SrO,0% or more but less than 8% of CaO, 0% or more but less than 13% of MgO,provided that the total content of BaO, SrO, CaO and MgO is 0 to 40%, 0to 20% of Gd₂O₃, 0 to 15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more butless than 0.5% of WO₃, 0% or more but less than 1.5% of a total of Na₂O,K₂O and Li₂O, 0 to 10% of GeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0to 10% of Al₂O₃, 0% or more but less than 2% of Sb₂O₃ and 0 to 1% ofSnO₂, and has a refractive index of 1.8 to 1.86, wherein the opticalglass exhibits λ₇₀ at 460 nm or less.

(5) The optical glass of above (1), which contains 2 to 45% of B₂O₃, 0to 30% of SiO₂, provided that the content of B₂O₃>the content of SiO₂),10 to 50% of La₂O₃, 0 to 30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂,0 to 35% of Nb₂O₅, 0 to 35% of BaO, 0% or more but less than 1% of SrO,0% or more but less than 8% of CaO, 0% or more but less than 13% of MgO,provided that the total content of BaO, SrO, CaO and MgO is 0 to 40%, 0to 20% of Gd₂O₃, 0 to 15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more butless than 0.5% of WO₃, 0% or more but less than 1.5% of a total of Na₂O,K₂O and Li₂O, 0 to 10% of GeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0to 10% of Al₂O₃, 0% or more but less than 2% of Sb₂O₃ and 0 to 1% ofSnO₂, and has a refractive index of 1.8 to 2.1, wherein the opticalglass exhibits λ₇₀ at 460 nm or less.

(6) The optical glass of above (1), which contains 2 to 45% of B₂O₃, 0to 30% of SiO₂, provided that the content of B₂O₃>the content of SiO₂,10 to 50% of La₂O₃, 0 to 30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂,0 to 35% of Nb₂O₅, 0 to 35% of BaO, 0 to 0.8% of SrO, 0 to 7% of CaO, 0to 12% of MgO, provided that the total content of BaO, SrO, CaO and MgOis 0 to 40%, 0 to 20% of Gd₂O₃, 0 to 15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0to 0.4% of WO₃, 0 to 1.2% of a total of Na₂O, K₂O and Li₂O, 0 to 10% ofGeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0 to 10% of Al₂O₃, 0 to 1.8%of Sb₂O₃ and 0 to 1% of SnO₂, wherein the optical glass exhibits λ₇₀ at460 nm or less.

(7) The optical glass of above (1), which contains 2 to 45% of B₂O₃, 0to 30% of SiO₂, provided that the content of B₂O₃>the content of SiO₂),10 to 50% of La₂C₃, 0 to 30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂,0 to 35% of Nb₂C₅, 0 to 35% of BaO, 0% or more but less than 1% of SrO,0% or more but less than 8% of CaO, 0% or more but less than 13% of MgO,provided that the total content of BaO, SrO, CaO and MgO is 0 to 40%, 0to 20% of Gd₂C₃, 0% or more but less than 2% of Y₂O₃, 0 to 18% of Ta₂C₅,0% or more but less than 0.5% of WO₃, 0% or more but less than 1.5% of atotal of Na₂C, K₂O and Li₂C, 0 to 10% of GeO₂, 0 to 20% of Bi₂C₃, 0 to10% of Yb₂C₃, 0 to 10% of Al₂O₃, 0% or more but less than 2% of Sb₂C₃and 0 to 1% of SnO₂.

(8) The optical glass of above (7), which contains 3 to 24% of B₂O₃, 0to 18% of SiO₂, provided that the weight ratio of the content ofB₂O₃/the content of SiO₂ is at least 1.1 or that no SiO₂ is contained,18 to 47% of La₂C₃, 0 to 26% of TiO₂, 0 to 12% of ZnO, 0 to 10% of ZrO₂,0 to 30% of Nb₂C₅, 0 to 32% of BaO, 0 to 10% of Gd₂C₃ and 0 to 4% ofYb₂C₃.

(9) The optical glass of above (7) or (8), which contains 1 to 5% ofZnO.

(10) The optical glass of above (1), wherein the total content of B₂O₃,SiO₂, La₂O₃, ZnO, ZrO₂, Nb₂O₅, TiO₂, BaO, CaO, SrO, Gd₂O₃, Y₂O₃, Ta₂O₅,WO₃, Na₂O, K₂O, Li₂O, GeO₂, Yb₂O₃, Sb₂O₃ and SnO₂ is 99% or more.

(11) The optical glass of above (10), wherein the total content of B₂O₃,SiO₂, La₂O₃, ZnO, ZrO₂, Nb₂O₅, TiO₂, BaO and Sb₂O₃ is 99% or more.

(12) The optical glass of above (11), which contains all of B₂O₃, SiO₂,La₂O₃, ZnO, ZrO₂, Nb₂O₅, TiO₂ and BaO.

(13) The optical glass of above (1), which contains TiO₂.

(14) A press-molding glass gob which is formed of the optical glassrecited in above (1) and is to be softened under heat and press-molded.

(15) An optical element formed of the optical glass recited in above(1).

According to the present invention, there is provided an optical glasswhose refractive index is high and whose coloring is reduced.

According to the present invention, further, there is provided apress-molding glass gob for producing, by press-molding, an opticalelement formed of an optical glass whose refractive index is high andwhose coloring is decreased.

Further, according to the present invention, there is provided anoptical element formed of an optical glass whose refractive index ishigh and whose coloring is decreased.

PREFERRED EMBODIMENTS OF THE INVENTION

The optical glass of the present invention comprises, by weight %, 2 to45% of B₂O₃, 0 to 30% of SiO₂ provided that the content of B₂O₃>thecontent of SiO₂, 10 to 50% of La₂O₃, 0 to 30% of TiO₂, 0 to 15% of ZnO,0 to 15% of ZrO₂, 0 to 35% of Nb₂O₅, 0 to 35% of BaO, 0 to 5% of SrO, 0%or more but less than 8% of CaO, 0% or more but less than 13% of MgO,provided that the total content of BaO, SrO, CaO and MgO is 0 to 40%, 0to 20% of Gd₂O₃, 0 to 15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more butless than 0.5% of WO₃, 0% or more but less than 1.5% of a total of Na₂O,K₂O and Li₂O, 0 to 10% of GeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0to 10% of Al₂O₃, 0% or more but less than 2% of Sb₂O₃ and 0 to 1% ofSnO₂.

According to the above optical glass, a high transmittance can beobtained in a visible light region, and particularly, a hightransmittance can be obtained in the short wavelength region of thevisible light region.

Further, there can be obtained an optical glass that is more stabilizedin the ranges of a refractive index (nd) of 1.8 to 2.1 and an Abbe'snumber (νd) of 20 to 40.

The above compositional ranges will be explained in detail below. Thefollowing content by % in each component below represents weight %.

B₂O₃ is a component effective as an oxide for forming a glass network ofthe above glass and is also effective for decreasing the temperature formeltability and flow viscosity of the glass, and at least 2% of B₂O₃ isrequired. However, the content of B₂O₃ exceeds 45%, the refractive indexdecreases. The content of B₂O₃ is therefore limited to 2 to 45%, and itis preferably 3 to 24%, more preferably 5 to 18%.

SiO₂ works to maintain the devitrification resistance of the aboveglass. When incorporated, SiO₂ works as a component for forming a glassnetwork. However, when the content of SiO₂ exceeds 30%, the meltabilityof the glass is degraded, and it is difficult to produce the glassstably. The content of SiO₂ is therefore limited to 0 to 30%, and it ispreferably 0 to 18%, more preferably 1 to 18%.

Further, when the content of B₂O₃ is smaller than the content of SiO₂,the glass comes to be easily colored, and the glass is degraded inmeltability and devitrification resistance, so that more B₂O₃ isincorporated than SiO₂.

La₂O₃ is an essential component for obtaining a high-refractivitylow-dispersion glass. When the content of La₂O₃ is less than 10%, therefractive index decreases, and when it is larger than 50%, thedevitrification resistance decreases, so that it is difficult to obtaina glass that can be stably produced. The content of La₂O₃ is thereforelimited to 10 to 50%, and it is preferably 18 to 47%, more preferably 25to 47%.

TiO₂ is a component for improving the glass in chemical durability anddevitrification resistance while adjusting optical properties such as arefractive index and an Abbe's number. For imparting the glass with theabove properties, it is required to incorporate 0 to 30% of TiO₂, andthe content thereof is preferably 0 to 26%, more preferably 1 to 26%,still more preferably 8 to 26%.

ZnO is a component for imparting the glass with high refractivity andlow-dispersion properties (the degree of dependency of the refractiveindex on wavelengths is small), and it is also a component for improvingthe glass in devitrification resistance and decreasing the temperaturefor viscous flowability. When the content of ZnO is larger than 15%, thedegree of devitrification increases, and it is difficult to obtain aglass that can be stably produced. The content of ZnO is thereforelimited to 0 to 15%, and it is preferably 0 to 12%. When a proper amountof ZnO is added, the spectral transmittance on the short wavelength endsharply rises. Therefore, the content of ZnO is preferably more than 0%,but not more than 5%, more preferably 0.5 to 5%, still more preferably 1to 5%.

ZrO₂ is a component for producing a high refractive index, and whenadded in a small amount, it has an effect that the glass is improved indevitrification resistance. However, when the content thereof exceeds15%, the devitrification resistance decreases, and the meltability ofthe glass is also degraded. The content of ZrO₂ is therefore limited to0 to 15%, and it is preferably 0 to 10%, more preferably 1 to 10%.

N₂O₅ is a component for imparting the glass with a high refractiveindex, and it also has an effect that the glass is improved indevitrification. The content of Nb₂O₅ is properly 0 to 35%. When thecontent of Nb₂O₅ exceeds 35%, the absorption on the short wavelengthside is intensified, and the coloring tendency is intensified. Thecontent of Nb₂O₅ is preferably 0 to 30%, more preferably 1 to 30%, stillmore preferably 1 to 20%, further more preferably 1 to 15%.

When used in the form of carbonate or nitrate as raw materials, BaO,SrO, CaO and MgO have an effect that the defoaming of the glass ispromoted.

When added in an amount of 0 to 35%, BaO has an effect that the glass isimproved in coloring. When the content of BaO exceeds 35%, however, thedevitrification resistance is degraded. The content of BaO is preferably0 to 32%, more preferably 1 to 32%, still more preferably 1 to 25%.

SrO may be added in an amount of 0 to 5% as a substitute for BaO.Similarly, 0% or more but less than 8% of CaO and 0% or more but lessthan 13% of MgO may be added. SrO can improve the glass indevitrification resistance when the glass is re-heated and molded, sothat the content of SrO is preferably 0% or more but less than 1%.Particularly, when the refractive index (nd) is 1.8 to 1.86, it isrequired to take care of a decrease in the above devitrificationresistance. When the refractive index (nd) is 1.8 to 1.86, preferably,the content of SrO is adjusted to 0% or more but less than 1%, and whenrefractive index (nd) is more than 1.86 to 2.1, preferably, the contentof SrO is adjusted to 0 to 5%. When the refractive index (nd) is 1.8 to2.1, more preferably, the content of SrO is adjusted to 0 to 0.8%.

Further, when the total content of BaO, SrO, CaO and MgO exceeds 40%,the glass is degraded in devitrification resistance, and it is difficultto obtain a glass that can be stably produced. The total content of BaO,SrO, CaO and MgO is therefore limited to 0 to 40%.

Gd₂O₃ can be added in an amount up to 20% as a substitute for La₂O₃.When the content of Gd₂O₃ exceeds 20%, the glass is degraded indevitrification resistance, and it is difficult to obtain a glass thatcan be stably produced. The content of Gd₂O₃ is therefore limited to 0to 20%, and it is preferably 0 to 10%.

Y₂O₃ and Yb₂O₃ can be also added in an amount of 0 to 15% and 0 to 10%,respectively, as substitutes for La₂O₃. However, when the contents ofthese components exceed the above upper limits, the glass is degraded indevitrification resistance, and it is difficult to obtain a glass thatcan be stably produced. Preferably, the content of Y₂O₃ is 0% or morebut less than 2%, and the content of Yb₂O₃ is 0 to 4%. The content ofY₂O₃ is more preferably in the range of 0 to 1.5%.

Ta₂O₅ is a component for imparting the glass with high-refractivity andlow-dispersion properties and is useful for forming a low-dispersionglass. However, when the content of Ta₂O₅ exceeds 18%, the glass isdegraded in meltability. The content of Ta₂O₅ is therefore properly 0 to18%.

WO₃ is a component for improving the glass in devitrification resistancewhen added in a small amount. However, when the content of WO₃ exceeds0.5%, the absorption of light in a short wavelength region by the glassis intensified, and the glass is strongly liable to be colored. Thecontent of WO₃ is therefore limited to 0% or more but less than 0.5%,and it is preferably 0 to 0.4%.

Na₂O, K₂O and Li₂O are components effective for decreasing the glasstransition temperature (Tg). Particularly, Li₂O has a remarkably higheffect on the above decrease. Since, however, these components cause theglass to suffer a large decrease in devitrification resistance andrefractive index, the total content of Na₂O, K₂O and Li₂O is thereforelimited to 0% or more but less than 1.5%.

GeO₂ has an effect similar to that of SiO₂, and it can be added in anamount up to 10%. When the content of GeO₂ exceeds 10%, thedevitrification resistance decreases. The content of GeO₂ is thereforeproperly 0 to 10%. However, the above optical glass can attain desiredproperties without GeO₂. Preferably, GeO₂ that is expensive is thereforenot incorporated.

When added in a small amount, Bi₂O₃ has an effect that the glasstransition temperature (Tg) is decreased. When the content of Bi₂O₃exceeds 20%, the devitrification resistance decreases, and it causes theglass to be colored. The content of Bi₂O₃ is therefore properly 0 to 20

When added in a small amount, Al₂O₃ sometimes works to improve the glassin devitrification resistance. However, the refractive index decreasesat the same time. The content of Al₂O₃ is therefore limited to 0 to 10%.

Ga₂O₃ and In₂O₃ can be also added in an amount up to approximately 10%.However, when added, they may degrade the devitrification resistance andthey are expensive materials. Desirably, therefore, Ga₂O₃ and In₂O₃ arenot incorporated.

In addition to the above components, Sb₂O₃ and SnO₂ that are generallyused as a clarifying agent may be added. The content of Sb₂O₃ is 0% ormore but less than 2%, and the content of SnO₂ is 0 to 1%.

However, As₂O₃ that strongly works as a clarifying agent has toxicity,so that it is desirable to add no As₂O₃.

In addition to the above oxides, desirably, lead, a lead compound andradioactive substances such as U and Th are not incorporated. Further,from the viewpoint of decreasing the coloring of the glass, it isnecessary to avoid the incorporation of substances that cause the glassto be colored, such as Cu, Cr, V, Fe, Ni and Co. Further, it isnecessary to avoid the addition of Te, Se and Cd.

In addition, the above JP-A-53-4023 describes an optical glasscontaining expensive HfO₂ as an essential component. In the presentinvention, however, the intended optical glass can be obtained withoutincorporating HfO₂.

Compositions having any combinations of preferred contents of the abovecomponents in the above explanations are preferred for obtaining desiredoptical glasses.

In the optical glass of the present invention, the refractive index (nd)and the Abbe's number (νd) are preferably in the range of 1.8 to 2.1(refractive index (nd)) and in the range of 20 to 40 (Abbe's number(νd)). The Abbe's number (νd) is more preferably in the range of 20 to39, and the refractive index (nd) is more preferably 1.81 to 2.1, stillmore preferably 1.85 to 2.1.

The transmittance property of the optical glass of the present inventionwill be explained below. The transmittance is quantitatively evaluatedas follows. First, a sheet glass having a thickness of 10 mm+0.1 mm isprepared. The sheet glass is formed of the above optical glass and hastwo surfaces that are lapped so as to be in parallel with each other.Light is allowed to perpendicularly enter the lapped surface of theabove sheet glass, and the sheet glass is measured for a spectraltransmittance including a surface reflection loss, in the wavelengthregion of 280 nm to 700 nm. A wavelength at which the spectraltransmittance comes to be 70% is taken as a wavelength λ₇₀, and awavelength at which the spectral transmittance comes to be 5% is takenas a wavelength λ5. In the wavelength region from 280 nm to 700 nm,preferably, only one single wavelength λ₇₀ and only one singlewavelength λ₅ are present. And, desirably, the optical glass exhibits aspectral transmittance of at least 5% in the entire wavelength regionfrom λ₅ to 700 and a spectral transmittance of at least 70% in theentire wavelength region from λ₇₀ to 700 nm.

When λ₇₀ and λ₅ are adjusted to be present on a shorter wavelength side,an optical glass having the above transmittance property comes toexhibit a higher transmittance in a broad range of the visible lightregion.

In the present invention, the optical glass is preferably an opticalglass that exhibits λ₇₀ at 460 nm or at a shorter wavelength, morepreferably an optical glass that exhibits λ₇₀ at 450 nm or at a shorterwavelength, still more preferably an optical glass that exhibits λ₇₀ at440 nm or at a shorter wavelength. For imparting the glass with variousproperties including the above refractive index and Abbe's number, morepreferably, λ₇₀ is in the range of 350 to 460 nm, still more preferably,λ₇₀ is in the range of 350 to 450 nm, further more preferably, λ₇₀ is inthe range of 350 to 440 nm.

In the present invention, the optical glass is preferably an opticalglass that exhibits λ₅ at 400 nm or at a shorter wavelength, morepreferably an optical glass that exhibits λ₅ at 390 nm or at a shorterwavelength. For imparting the glass with various properties includingthe above refractive index and Abbe's number, more preferably, λ₅ is inthe range of 300 to 390 nm.

Further, the optical glass of the present invention is further morepreferably an optical glass that exhibits λ₇₀ and λ₅ which satisfy theabove ranges at the same time.

Since λ₇₀ and λ₅ (particularly λ₇₀) are liable to change under glassmelting conditions, it is necessary to take account of the meltingtemperature and the melting time period so that λ₇₀ and λ₅ can be on ashorter wavelength side. It is also necessary to reduce impurities thatcause coloring.

Examples of more preferred compositions, preferred optical constants andpreferred ranges of λ₇₀ are as described below.

(Optical Glass 1)

An optical glass containing 2 to 45% of B₂O₃, 0 to 30% of SiO₂, providedthat the content of B₂O₃>the content of SiO₂, 10 to 50% of La₂O₃, 0 to30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂, 0 to 35% of Nb₂O₅, 0 to35% of BaO, 0 to 5% of SrO, 0% or more but less than 8% of CaO, 0% ormore but less than 13% of MgO, provided that the total content of BaO,SrO, CaO and MgO is 0 to 40%, 0 to 20% of Gd₂O₃, 0 to 15% of Y₂O₃, 0 to18% of Ta₂O₅, 0% or more but less than 0.5% of WO₃, 0% or more but lessthan 1.5% of a total of Na₂O, K₂O and Li₂O, 0 to 10% of GeO₂, 0 to 20%of Bi₂O₃, 0 to 10% of Yb₂O₃, 0 to 10% of Al₂O₃, 0% or more but less than2% of Sb₂O₃ and 0 to 1% of SnO₂, and having a refractive index of morethan 1.86 but up to 2.1, wherein the optical glass exhibits λ₇₀ at 460nm or less.

(Optical Glass 2)

An optical glass containing 2 to 45% of B₂O₃, 0 to 30% of SiO₂, providedthat the content of B₂O₃>the content of SiO₂), 10 to 50% of La₂O₃, 0 to30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂, 0 to 35% of Nb₂O₅, 0 to35% of BaO, 0% or more but less than 2% of SrO, 0% or more but less than8% of CaO, 0% or more but less than 13% of MgO, provided that the totalcontent of BaO, SrO, CaO and MgO is 0 to 40%, 0 to 20% of Gd₂O₃, 0 to15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more but less than 0.5% of WO₃, 0%or more but less than 1.5% of a total of Na₂O, K₂O and Li₂O, 0 to 10% ofGeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0 to 10% of Al₂O₃, 0% ormore but less than 2% of Sb₂O₃ and 0 to 1% of SnO₂, and having arefractive index of 1.8 to 1.86, wherein the optical glass exhibits λ₇₀at 460 nm or less.

(Optical Glass 3)

An optical glass containing 2 to 45% of B₂O₃, 0 to 30% of SiO₂, providedthat the content of B₂O₃>the content of SiO₂), 10 to 50% of La₂O₃, 0 to30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂, 0 to 35% of Nb₂O₅, 0 to35% of BaO, 0% or more but less than 1% of SrO, 0% or more but less than8% of CaO, 0% or more but less than 13% of MgO, provided that the totalcontent of BaO, SrO, CaO and MgO is 0 to 40%, 0 to 20% of Gd₂O₃, 0 to15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more but less than 0.5% of WO₃, 0%or more but less than 1.5% of a total of Na₂O, K₂O and Li₂O, 0 to 10% ofGeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0 to 10% of Al₂O₃, 0% ormore but less than 2% of Sb₂O₃ and 0 to 1% of SnO₂, and having arefractive index of 1.8 to 2.1, wherein the optical glass exhibits λ₇₀at 460 nm or less.

(Optical Glass 4)

An optical glass containing 2 to 45% of B₂O₃, 0 to 30% of SiO₂, providedthat the content of B₂O₃>the content of SiO₂), 10 to 50% of La₂O₃, 0 to30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂, 0 to 35% of Nb₂O₅, 0 to35% of BaO, 0 to 0.8% of SrO, 0 to 7% of CaO, 0 to 12% of MgO, providedthat the total content of BaO, SrO, CaO and MgO is 0 to 40%, 0 to 20% ofGd₂O₃, 0 to 15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0 to 0.4% of WO₃, 0 to 1.2%of a total of Na₂O, K₂O and Li₂O, 0 to 10% of GeO₂, 0 to 20% of Bi₂O₃, 0to 10% of Yb₂O₃, 0 to 10% of Al₂O₃, 0 to 1.8% of Sb₂O₃ and 0 to 1% ofSnO₂, wherein the optical glass exhibits λ₇₀ at 460 nm or less.

(Optical Glass 5)

An optical glass containing 2 to 45% of B₂O₃, 0 to 30% of SiO₂, providedthat the content of B₂O₃>the content of SiO₂), 10 to 50% of La₂O₃, 0 to30% of TiO₂, 0 to 15% of ZnO, 0 to 15% of ZrO₂, 0 to 35% of Nb₂O₅, 0 to35% of BaO, 0% or more but less than 1% of SrO, 0% or more but less than8% of CaO, 0% or more but less than 13% of MgO, provided that the totalcontent of BaO, SrO, CaO and MgO is 0 to 40%, 0 to 20% of Gd₂O₃, 0% ormore but less than 2% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more but lessthan 0.5% of WO₃, 0% or more but less than 1.5% of a total of Na₂O, K₂Oand Li₂O, 0 to 10% of GeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0 to10% of Al₂O₃, 0% or more but less than 2% of Sb₂O₃ and 0 to 1% of SnO₂.

(Optical Glass 6)

An optical glass that is included in the above optical glass 5 and whichcontains 3 to 24% of B₂O₃, 0 to 18% of SiO₂, provided that the weightratio of the content of B₂O₃/the content of SiO₂ is at least 1.1 or thatno SiO₂ is contained, 18 to 47% of La₂O₃, 0 to 26% of TiO₂, 0 to 12% ofZnO, 0 to 10% of ZrO₂, 0 to 30% of Nb₂O₅, 0 to 32% of BaO, 0 to 10% ofGd₂O₃ and 0 to 4% of Yb₂O₃.

(Optical Glass 7)

An optical glass that is included in the above optical glass 5 or 6 andwhich contains 1 to 5% of ZnO.

(Optical Glass 8)

An optical glass that is included in any one of the above opticalglasses 1 to 7 and wherein the total content of B₂O₃, SiO₂, La₂O₃, ZnO,ZrO₂, Nb₂O₅, TiO₂, BaO, CaO, SrO, Gd₂O₃, Y₂O₃, Ta₂O₅, WO₃, Na₂O, K₂O,Li₂O, GeO₂, Yb₂O₃, Sb₂O₃ and SnO₂ is 99% or more, more preferably 100%.

(Optical Glass 9)

An optical glass that is included in the above optical glass 8 and wherethe total content of B₂O₃, SiO₂, La₂O₃, ZnO, ZrO₂, Nb₂O₅, TiO₂, BaO andSb₂O₃ is 99% or more, more preferably 100%.

(Optical Glass 10)

An optical glass that is included in the above optical glass 9 and whichcontains all of B₂O₃, SiO₂, La₂O₃, ZnO, ZrO₂, Nb₂O₅, TiO₂ and BaO.

(Optical Glass 11)

An optical glass that is included in any one of the above opticalglasses 1 to 11 and which contains TiO₂. In this optical glass, theweight ratio of the content of Nb₂O₅/the content of TiO₂ is preferablyfrom 0 to 7, more preferably from 0 to 6. In this constitution, anoptical glass to be colored to less degree can be obtained.

Having the above transmittance property, the optical glass of thepresent invention exhibits a high transmittance to light in a shortwavelength region of the visible light region, so that a well-balancedimage-sensing optical system can be easily constituted. The opticalglass of the present invention is particularly suitable as a materialfor an optical element such as a lens for constituting an image-sensingsystem for a solid image-sensing device.

The liquidus temperature of the optical glass of the present inventionwill be explained below. In the optical glass of the present invention,the liquidus temperature is preferably 1,250° C. or lower, morepreferably 1,200° C. or lower. As far as the stability of the glass isconcerned, an optical glass that shows no liquidus temperature ispreferred. For imparting the glass with the above various properties,however, the optical glass of the present invention has a liquidustemperature of 900 to 1,200° C.

The press-molding glass gob of the present invention and the method forpreparation thereof will be explained below. The press-molding glass gobis a glass molded material that is to be softened under heat andpress-molded, and it is also called a press-molding preform (preform forpress-molding). The weight and form of the press-molding glass gob aredetermined as required depending upon a press-molded product as an endproduct. The press-molding glass gob of the present invention is formedof the above optical glass and has various properties that reflect thevarious properties of the optical glass of the present invention.

In the method for preparing the above press-molding glass gob, a moltenglass is shaped into a moldable glass gob formed of the above opticalglass. First, glass raw materials for obtaining the optical glass of thepresent invention are formulated, dissolved, clarified and homogenizedto obtain a homogeneous molten glass free of gas bubbles and foreignmatter. Then, the molten glass is flowed out from a flow pipe made of aplatinum alloy, or the like. For flowing the molten glass out, thetemperature of the flow pipe, etc., are arranged such that the glass isnot devitrified. The molten glass that flows out is cast into areceiving mold or a casting mold to shape the molten glass into apredetermined form. Methods suitable for the above shaping will bedescribed as examples.

The first shaping method is a method in which a plurality of receivingmolds are consecutively transferred into a place below the flow pipe toreceive a molten glass gob having a predetermined weight on eachreceiving mold and each glass gob is cooled while each is shaped. Inthis method, a forward end of the molten glass flow flowing out of thepipe is supported on a receiving mold, and the receiving mold is rapidlymoved down timely when a molten glass gob having a predetermined weightcan be separated. In this case, the supply of the molten glass onto thereceiving mold does not keep up with the downward movement of thereceiving mold, and a leading molten glass flow is separated from amolten glass on the backward side of the flow, so that a predeterminedweight of the molten glass gob can be received on the receiving mold. Inthis manner, a glass can be shaped without leaving a cut mark (shearmark) formed when a molten glass flow is cut with a blade. In the abovefirst method, a glass gob having a weight equivalent to, or a littlelarger than, a press-molding glass gob can be shaped.

When a glass gob having a weight equivalent to the weight of onepress-molding glass gob is shaped, this glass gob can be used as apress-molding glass gob. In this case, it is preferred to cool the glassgob at such a rate that the glass is not broken.

When a glass gob having a weight larger than the weight of onepress-molding glass gob is shaped, this glass gob is annealed to reducestrains and then machined to complete a glass gob having a weightequivalent to the weight of one press-molding glass gob, and thecompleted glass gob is used as a press-molding glass gob. According tothis method, glass gobs are prepared beforehand, and the glass gobs aremachined depending upon demands to adjust their weights, whereby therecan be supplied press-molding glass gobs that can be press-molded intooptical elements having various sizes. For the above machining, barrellapping is preferred.

Further, when the above press-molding glass gob is used for precisionpress-molding, a press-molding glass gob that is shaped without anymachining is preferred.

The second shaping method is a method in which a molten glass is castinto a casting mold at a constant speed, the casting mold having anearly horizontal bottom surface and a pair of sidewalls facing eachother in parallel across the bottom surface. A cast molten glassuniformly spreads within the casting mold to be shaped into a glasssheet having a width determined by the distance of the above pair ofsidewalls. The thus-formed glass sheet is drawn out from an openingportion of the casting mold at a speed depending upon a molten glasssupply speed such that a sheet having a uniform thickness and a uniformwidth can be obtained. The thus-obtained glass sheet is annealed toreduce strains and then cut to a predetermined size. The thus-obtainedglass piece is called “cut piece”. The cut piece is chamfered asrequired or machined so as to have a weight equivalent to the weight toa press-molding glass gob. For chamfering the cut piece or machining thecut piece for adjusting its weight, barrel lapping is preferred.

In any one of the above methods, there can be obtained the press-moldingglass gob of the present invention, which has a predetermined weight andis formed of the optical glass of the present invention. In addition, amold release film may be formed on the press-molding glass gob, or apowdered mold release agent may be applied thereto, as required formaking it easy to separate a molded product from a mold when thepress-molding is carried out. However, the powdered mold release agentis undesirable for precision press-molding, since the mold release agentis transferred to the glass forming the glass gob (press-moldedproduct).

The optical element of the present invention will be explained below.The optical element of the present invention is formed of the aboveoptical glass of the present invention. The optical element thereforehas various properties of the above optical glass. As a typicalembodiment, the optical element of the present invention has arefractive index (nd) of 1.8 to 2.1 and an Abbe's number (νd) of 20 to40, and also commonly has the property of a high transmittance on ashort wavelength side of the visible light region. The optical elementformed of the above optical glass preferably exhibits λ₇₀ and λ₅ in theabove-described ranges, exhibits a high transmittance to visible lightand is free from coloring. This optical element is suitable for opticalsystems of cameras using a solid image-sensing device such as a digitalcamera, a video camera and a camera incorporated into a mobile item.

The optical element of the present invention includes various lensessuch as a spherical lens, an aspherical lens, a microlens and a lensarray, a prism and a diffraction grating. The optical element of thepresent invention may be provided with an optical thin film such as ananti-reflection film, a partial reflection film, a high reflection filmor the like as required.

The method for producing the optical element of the present inventionwill be explained below. In the method for producing the optical elementof the present invention, the above press-molding glass gob or apress-molding glass gob prepared by the above preparation method issoftened under heat and press-molded with a press mold, to produce theoptical element.

The optical element has an optical-function surface having an opticalfunction, which refracts, transmits, diffract or reflects light. Thepress-molding method can be largely classified into the following twomethods depending upon how the above optical-function surface is formed.

The first method is a method in which a press-molded product having aform similar to an end optical element and having a larger size than theoptical element is formed by press-molding. The press-molded product ispolished and/or lapped, and the surface of the optical element includingthe optical-function surface is formed by machining. Since the machiningis carried out after the press-molding, it is preferred to anneal thepress-molded product to decrease strains in order to prevent thebreaking of the glass during the machining. In this method, thepress-molding can be carried out in atmosphere, so that the abovepowdered mold release agent may be used.

The second method is a so-called precision press-molding method, inwhich the molding surface of a press mold is precisely worked so as tohave an inversion form of the form of an end optical element, a moldrelease film is optionally formed, and the form of the above moldingsurface is precisely transferred, by press molding, to a glass gob thatis softened under heat. According to this method, the optical-functionsurface can be formed by the press-molding without polishing or lapping.However, it is required to carry out the press-molding in anon-oxidizing atmosphere such as a nitrogen gas atmosphere.

In the above second method, it is not essential to machine thepress-molded product, so that a strain may remain so long as the strainhas no optical influence, and the annealing of the press-molded productcan be omitted. As a method for producing an optical element, further,there is another method in which a glass in a molten state is fed into apress mold to produce a press-molded product having a form similar tothe form of an optical element, and the press-molded product is polishedand lapped to complete an optical element.

The refractive index (nd) and the Abbe's number (νd) of the opticalelement slightly change due to a thermal history during the process ofproducing the optical element. For producing an optical element havingprecisely determined optical constants, the glass composition and thethermal history during the production process can be adjusted by takingaccount of the above change in refractive index (nd) and Abbe's number(νd).

In this manner, there can be provided an optical element that haspredetermined optical constants and an excellent transmittance and whichis particularly suitable as an optical part for a machine, equipment oran item on which a solid image-sensing device, or the like is mounted.

EXAMPLES

The present invention will be explained further in detail with referenceto Examples hereinafter, while the present invention shall not belimited by these Examples.

Examples 1-11

A raw material batch formulated to give 100 g of a glass having acomposition shown in Table 1 or 2 was placed in a crucible formed ofplatinum and melted in a furnace set at 1,300° C., and a molten glass isstirred, clarified, then cast into a casting mold made of iron,maintained at a temperature around a glass transition temperature (Tg)for 2 hours and gradually cooled, to give an optical glass.

The thus-obtained optical glass in each Example was measured for arefractive index (nd), an Abbe's number (νd), a liquidus temperature(LT) and λ₇₀ and λ₅ as follows. Tables 1 and 2 show the results.

(1) Refractive Index (nd) and Abbe's Number (νd)

An optical glass obtained by cooling at a temperature-decrease rate of−30° C./h was measured.

(2) Liquidus Temperature (LT)

A plurality of crucibles made of platinum were prepared, and 50 cm³ ofglass was placed in each crucible. The crucibles with the glass in themwere placed in furnaces in which the temperature was set at intervals of10° C. and held under different temperature conditions for 2 hours.After cooled, an inside of each glass was observed through a microscopehaving a magnification of 100 times for the presence or absence of acrystal, on the basis of which the liquidus temperature was determined.

(3) λ₇₀ and λ₅

A 10 mm thick lapped sample was measured for spectral transmittances,and a wavelength (nm) at which the sample exhibited a transmittance of5% was determined as λ₅ and a wavelength (nm) at which the sampleexhibited a transmittance of 70% was determined as λ₇₀.

TABLE 1 Examples 1 2 3 4 5 6 Glass B₂O₃ 13.16 8.20 13.16 13.04 10.5013.00 composition SiO₂ 6.63 6.00 6.63 6.57 6.50 10.00 (wt %) La₂O₃ 19.0349.00 36.19 35.85 34.00 37.00 ZnO 7.21 0.00 7.21 2.68 3.00 3.00 ZrO₂5.68 5.40 7.05 4.28 7.00 7.00 Nb₂O₅ 21.59 0.00 3.85 6.76 7.00 4.00 TiO₂3.94 4.40 9.04 12.44 16.00 9.00 BaO 21.94 0.00 16.86 18.38 16.00 17.00CaO 0.00 0.00 0.00 0.00 0.00 0.00 SrO 0.00 0.00 0.00 0.00 0.00 0.00 (RO)(21.94) (0.00) (16.86) (18.38) (16.00) (17.00) Gd₂O₃ 0.00 0.00 0.00 0.000.00 0.00 Y₂O₃ 0.00 1.00 0.00 0.00 0.00 0.00 Ta₂O₅ 0.00 18.00 0.00 0.000.00 0.00 WO₃ 0.30 0.00 0.00 0.00 0.00 0.00 Na₂O 0.00 0.00 0.00 0.000.00 0.00 K₂O 0.52 0.00 0.00 0.00 0.00 0.00 Li₂O 0.00 0.00 0.00 0.000.00 0.00 (R₂O) (0.52) (0.00) (0.00) (0.00) (0.00) (0.00) GeO₂ 0.00 5.000.00 0.00 0.00 0.00 Bi₂O₃ 0.00 0.00 0.00 0.00 0.00 0.00 Yb₂O₃ 0.00 3.000.00 0.00 0.00 0.00 Total 100.00 100.00 100.00 100.00 100.00 100.00 P.Properties nd 1.8741 1.9225 1.87247 1.89803 1.94875 1.85403 νd 31.2735.95 34.21 31.14 28.25 34.95 λ₇₀ (nm) 397 405 402 407 418 391 λ₅ (nm)353 364 352 360 364 353 LT (° C.) 1160 1050 1080 1100 1150 1090 P.Properties = Physical Properties

TABLE 2 Examples 7 8 9 10 11 Glass B₂O₃ 8.00 12.70 23.00 13.2 8.0composition SiO₂ 6.00 6.30 0.00 6.6 6.0 (wt %) La₂O₃ 34.00 33.60 40.0036.2 34.0 ZnO 2.50 2.50 5.00 2.7 2.0 ZrO₂ 6.50 6.50 5.00 5.7 6.5 Nb₂O₅8.50 2.00 17.00 5.4 8.0 TiO₂ 19.00 5.00 5.00 13.4 20.5 BaO 15.50 31.400.00 16.8 15.0 CaO 0.00 0.00 0.00 0.0 0.0 SrO 0.00 0.00 0.00 0.0 0.0(RO) (15.50) (31.40) (0.00) (16.8) (15.0) Gd₂O₃ 0.00 0.00 0.00 0.0 0.0Y₂O₃ 0.00 0.00 0.00 0.0 0.0 Ta₂O₅ 0.00 0.00 5.00 0.0 0.0 WO₃ 0.00 0.000.00 0.0 0.0 Na₂O 0.00 0.00 0.00 0.0 0.0 K₂O 0.00 0.00 0.00 0.0 0.0 Li₂O0.00 0.00 0.00 0.0 0.0 (R₂O) (0.00) (0.00) (0.00) (0.0) (0.0) GeO₂ 0.000.00 0.00 0.0 0.0 Bi₂O₃ 0.00 0.00 0.00 0.0 0.0 Yb₂O₃ 0.00 0.00 0.00 0.00.0 Total 100.00 100.00 100.00 100.0 100.0 P. Properties nd 1.99451.82546 1.90564 1.90047 2.00030 νd 25.88 38.6 31.71 30.71 25.51 λ₇₀ (nm)437 407 424 408 440 λ₅ (nm) 370 341 362 362 372 LT (° C.) 1150 1090 10501080 1150 P. Properties = Physical Properties

Tables 1 and 2 show that the optical glasses of the present inventionshown in Examples had a refractive index (nd) in the range of 1.8 to 2.1and an Abbe's number (νd) in the range of 20 to 40.

Example 12

Each of the clarified and homogenized molten glasses obtained from theglasses in Examples 1 to 11 was independently cast from a pipe formed ofplatinum into a casting mold having one sidewall opened, at a constantflow rate, and while a glass sheet having a constant thickness and aconstant width was formed, the glass sheet was drawn out of the openingportion of the casting mold. The glass sheet that was drawn out wasannealed in an annealing furnace to decrease strains. In this manner,there were obtained glass sheets of the optical glasses of the aboveExamples 1 to 11, which had strains decreased and were homogenous,colorless and free of foreign matter.

Each glass sheet was cut into small cubes to obtain a plurality of cutpieces having identical dimensions. Further, a plurality of the cutpieces were barrel lapped so as to have an intended weight, and used aspress-molding glass gobs.

Besides the above method, there may be employed a method in which theabove molten glass is flowed out from a nozzle formed of platinum at aconstant speed, many receiving molds are transferred into a place belowthe nozzle one after another, to receive a molten glass gob on eachreceiving mold, each molten glass gob is shaped into a spherical form ora sphere-flattened form, annealed, then barrel-lapped to adjust theweight of each to an intended weight, and the thus formed glass gobs areused as press-molding glass gobs.

Example 13

A powdered mold release agent was applied to the entire surface of eachglass gob obtained in Example 12, and each glass gob was independentlysoftened under heat with a heater and then charged into a press moldhaving an upper mold member and a lower mold member. The glass gobs wererespectively pressed with the press mold to give lens blanks having theform of a lens each.

Then, the lens blanks were annealed to remove strains and adjust theirrefractive indexes and Abbe's numbers to predetermined values. Thecooled lens blanks were polished and lapped to produce lenses. The abovesteps in series were carried out in atmosphere.

The thus-obtained lenses were excellent in transmittance properties andhad various properties of the optical glasses of Examples 1 to 11. Ananti-reflection film may be formed on each lens as required.

The above lenses can constitute an excellent image-sensing opticalsystem.

INDUSTRIAL UTILITY

The optical glass of the present invention has a high refractive indexand has its coloring decreased, so that it can be suitably used in animage-sensing unit such as a digital camera using CCD as animage-sensing device.

1. A process for producing an optical element, which comprises softeninga press-molding glass gob under heat and press-molding it, saidpress-molding glass gob consisting of an optical glass comprising, byweight %, 2 to 45% of B₂O₃, 0 to 30% of SiO₂ provided that the contentof B₂O₃>the content of SiO₂, 10 to 50% of La₂O₃, 0 to 30% of TiO₂, 0 to15% of ZnO, 0 to 15% of ZrO₂, 0 to 35% of Nb₂O₅, 0 to 35% of BaO, 0 to5% of SrO, 0% or more but less than 8% of CaO, 0% or more but less than13% of MgO, provided that the total content of BaO, SrO, CaO and MgO is0 to 40%, 0 to 20% of Gd₂O₃, 0 to 15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% ormore but less than 0.5% of WO₃, 0% or more but less than 1.5% of a totalof Na₂O, K₂O and Li₂O, 0 to 10% of GeO₂, 0 to 20% of Bi₂O₃, 0 to 10% ofYb₂O₃, 0 to 10% of Al₂O₃, 0% or more but less than 2% of Sb₂O₃ and 0 to1% of SnO₂ and having an Abbe's number (νd) of 20 to
 40. 2. The processfor producing an optical element as recited in claim 1, wherein saidoptical glass has a refractive index (nd) of 1.8 to 2.1.
 3. The processfor producing an optical element as recited in claim 1, wherein saidoptical glass has a glass composition in which B₂O₃, SiO₂, La₂O₃, TiO₂,ZrO₂, Nb₂O₅ and BaO are co-present, comprises, by weight %, 2 to 45% ofB₂O₃, 1 to 18% of SiO₂ provided that the content of B₂O₃>the content ofSiO₂, 10 to 50% of La₂O₃, 1 to 26% of TiO₂, 0 to 15% of ZnO, 1 to 10% ofZrO₂, 1 to 30% of Nb₂O₅, 1 to 32% of BaO, 0 to 5% of SrO, 0% or more butless than 8% of CaO, 0% or more but less than 13% of MgO, provided thatthe total content of BaO, SrO, CaO, and MgO is 1 to 40%, 0 to 20% ofGd₂O₃, 0 to 15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more but less than0.5% of WO₃, 0% or more but less than 1.5% of a total of Na₂O, K₂O andLi₂O, 0 to 10% of GeO₂, 0 to 20% of Bi₂O₃, 0 to 10% of Yb₂O₃, 0 to 10%of Al₂O₃, 0% or more but less than 2% of Sb₂O₃ and 0 to 1% of SnO₂ andhas a refractive index (nd) of 1.8 to 2.1.
 4. The process for producingan optical element as recited in claim 1, wherein said optical glasscomprises, by weight %, 2 to less than 18% of B₂O₃, 0 to 18% of SiO₂provided that the weight ratio of the content of B₂O₃/the content ofSiO₂ is at least 1.1 or or that no SiO₂ is contained, 10 to 50% ofLa₂O₃, 1 to 30% of TiO₂, 0 to 15% of ZnO, 1 to 15% of ZrO₂, 1 to 35% ofNb₂O₅, 1 to 35% of BaO, 0 to 5% of SrO, 0% or more but less than 8% ofCaO, 0% or more but less than 13% of MgO, provided that the totalcontent of BaO, SrO, CaO, and MgO is 1 to 40%, 0 to 20% of Gd₂O₃, 0 to15% of Y₂O₃, 0 to 18% of Ta₂O₅, 0% or more but less than 0.5% of WO₃, 0%or more but less than 1.5% of a total of Na₂O, K₂O and Li₂O, 0 to 20% ofBi₂O₃, 0 to 10% of Yb₂O₃, 0 to 10% of Al₂O₃, 0% or more but less than 2%of Sb₂O₃ and 0 to 1% of SnO₂.
 5. The process for producing an opticalelement as recited in claim 4, wherein said optical glass comprises, byweight %, 1 to less than 18% of SiO₂, 1 to 26% of TiO₂, 1 to 30% ofNb₂O₅ and 1 to 32% of BaO.
 6. The process for producing an opticalelement as recited in claim 4, wherein said optical glass comprises, byweight %, more than 0% but not more than 5% of ZnO.
 7. The process forproducing an optical element as recited in claim 4, wherein said opticalglass comprises 1 to 15% by weight of Nb₂O₅.
 8. The process forproducing an optical element as recited in claim 1 or 3, wherein saidoptical glass is free of GeO₂.
 9. The process for producing an opticalelement as recited in claim 1 or 3, wherein said optical glass has arefractive index (nd) of 1.81 to 2.1.
 10. The process for producing anoptical element as recited in claim 9, wherein said optical glass has arefractive index (nd) of 1.85 to 2.1.
 11. The process for producing anoptical element as recited in claim 1 or 3, wherein said optical glassexhibits, as a transmittance property, λ₇₀ at 460 nm or at a shorterwavelength, the λ₇₀ being a wavelength determined by preparing, from theoptical glass, a sheet glass having a thickness of 10 mm±0.1 mm andhaving two surfaces that are lapped so as to be in parallel with eachother, causing light to perpendicularly enter the lapped surface of thesheet glass, measuring the sheet glass for a spectral transmittanceincluding a surface reflection loss, in the wavelength region of 280 nmto 700 nm, and determining the wavelength at which the spectraltransmittance comes to be 70%.
 12. The process for producing an opticalelement as recited in claim 1 or 3, wherein said optical glass exhibits,as a transmittance property, λ₅ at 400 nm or at a shorter wavelength,the λ₅ being a wavelength determined by preparing, from the opticalglass, a sheet glass having a thickness of 10 mm±0.1 mm and having twosurfaces that are lapped so as to be in parallel with each other,causing light to perpendicularly enter the lapped surface of the sheetglass, measuring the sheet glass for a spectral transmittance includinga surface reflection loss, in the wavelength region of 280 nm to 700 nm,and determining the wavelength at which the spectral transmittance comesto be 5%.